Why is no operating temperature range specified?

Discrete semiconductor devices have no concept of operating temperature. This is because, the current flowing through the element and the applied voltage differ depending on the customer's circuit and conditions, and the amount of heat generated cannot be defined by the ambient temperature and power supply voltage. For this reason, in discrete devices, instead of the operating temperature range, the absolute maximum rated channel temperature (or junction temperature) is specified for the upper limit of the device temperature, and the storage temperature is specified for the lower limit. Please use within this temperature range.